1. Field of the Invention
The present invention relates to a recording and/or reproducing apparatus for a disc-shaped optical recording medium, and a photodetector and an optical head employed for the recording and/or reproducing apparatus. More particularly, the present invention relates to a recording and/or reproducing apparatus for a disc-shaped optical recording medium, which may be a read-only optical recording medium or a magneto-optical recording medium on which information signals may be re-recorded, and a photodetector and an optical head employed for the recording and/or reproducing apparatus.
2. Background of the Invention
The optical recording medium on or from which information signals are recorded or reproduced by a light beam, referred to hereinafter as an optical disc, may be classified into a read-only optical disc, known as a so-called compact disc, and a re-recordable optical disc which permits not only recording but also recording and erasure of information signals.
With the read-only optical disc, a pattern of microirregularities carrying information signals is formed on a concentric or spirally extending recording track(s) on one surface of the disc. Specifically, the optical disc includes a disc-shaped substrate, a reflective layer and a protective layer. The disc-shaped substrate is formed of a light-transmitting synthetic material, such as polycarbonate or PMMA. A reflective layer of a metal, such as Al or Au, covers phase pits in a pattern of the microirregularities on one of the surfaces of the disc, substrate. A protective layer covers and protects the reflective layer.
As the re-recordable optical disc, there are presently known an optical disc formed of a phase-change type optical recording material and a magneto-optical disc formed of a vertical magnetic recording magnetic material. In general, the latter disc, that is the magneto-optical disc, has found practical application.
The magneto-optical disc has a disc-shaped substrate, a recording layer, and a protective layer. The disc-shaped substrate is formed of a light-transmitting synthetic material, such as polycarbonate or PMMA and forms a guide groove for guiding the light beam on its one surface. The recording layer is formed of a vertical magnetic recording material, such as Te, Fe or Co, for covering the guide groove. The protective layer covers and protects the recording layer.
For reproducing the read-only optical disc, a light beam radiated from a laser light source is irradiated to the disc substrate after being converged by an objective lens. The light beam reflected by the reflective layer is received by a photodetector through the objective lens. The readout signals of the information signals recorded on the read-only optical disc may be produced by taking advantage of the diffraction by the phase pits of the optical disc of the light beam reflected back by the reflective layer on the optical disc.
With the re-recordable disc, above all, the magneto-optical disc, a light beam radiated from a laser light source is irradiated to the disc substrate after being converged by an objective lens, as in the read-only disc mentioned above. The light beam reflected by the recording layer is received by a photodetector through the objective lens. The readout signals of the information signals recorded on the magneto-optical disc may be reproduced by detecting the Kerr rotation angle in the light beam reflected by the recording layer of the magneto-optical disc.
The arrangement of a conventional optical head for reproducing information signals from the magneto-optical disc includes an optical head 105 made up of a semiconductor laser 101, an objective lens 102 and first and second photodetectors 103, 104, as shown in FIG. 1. The optical head 105, arranged as a unit, is movable along the radius of a magneto-optical disc 106.
A light beam L radiated from the semiconductor laser 101 is collimated by a collimator lens 107 so as to be incident on a diffraction grating 108. The light beam L, divided into at least three beams, is transmitted through a boundary plane 109a of a beam splitter 109 and thence directed to the object lens 102. The objective lens 102 causes the three beams divided from the light beam L incident thereon to be converged and radiated on the recording layer of the magneto-optical disc 106. A central one of the three beams divided from the light beam L is radiated on the center region of the recording track on the magneto-optical disc 106. The remaining two side beams are disposed on both sides of the center light beam and irradiated on the guide groove. The center light beam L radiated on the center region of the recording track has its plane of polarization rotated responsive to the pattern of magnetization of the vertical magnetic recording layer on the recording track. The side light beam L radiated on the guide groove are modulated depending on the edge of the guide groove.
Three reflected light beam Lr reflected by the magneto-optical disc 106 are incident on the beam splitter 109 via the objective lens 102 and reflected at the boundary plane 109a. The three reflected light beams Lr, reflected by the boundary plane 109a, undergo total reflection at a reflective surface 109b, formed at an end of the beam splitter 109, and has its direction of polarization rotated by 45.degree. by a downstream side a half-wave plate 110.
The reflected light beam Lr transmitted through the half-wave plate 110 are incident on a polarization beam splitter 113 of an analyzer via a multiple lens 112 made up of a converging lens 111, a concave lens and a cylindrical lens. The three reflected light beam Lr incident on the polarization beam splitter 113, having a P-polarization component, are transmitted through the boundary plane 113a so as to be incident on a first photodetector 103. Three reflected light beam having an S-polarization component are reflected by the boundary plane 109a so as to be incident on a second photodetector 104. The multiple lens 112 plays the role of adjusting the focal distance of the reflected light beams Lr incident on the first and second photodetectors 103, 104 and for producing astigmatism.
The first photodetector 103 is made up of plural photodetectors, that is a central four-segment photodetectors A, B, C, D and two-segment photodetectors E, F and G, H on both sides of the photodetectors A to D, with the division lines of the photodetectors E, F and G, H extending radially of the optical disc. The second photodetector unit 104 includes a four-segment photodetectors a, b, c and d. Outputs of the photodetectors are supplied to a processing circuit, not shown.
The processing circuit calculates an information signal Si corresponding to information signals recorded on the magneto-optical disc 106, a focusing error signal Sf and a tracking error signal St in accordance with the equations (1) EQU Si=(A+B+C+D)-(a+b+c+d) EQU Sf=(A+C)-(B+D) (1) EQU St=[(A+D)-(B+C)]-K[(E-F)-(G-H)]
Of these signals, the information signal Si is supplied to a downstream side signal processing circuit, not shown, and the two error signals Sf and St are supplied to an actuator 114.
The focusing error signal Sf is based on the so-called astigmatic method, whereas the tracking error signal St is based on the push-pull principle. As for the astigmatic method and the push-pull method, reference is had to e.g. the U.S. Pat. Nos. 4,023,033 and 3,909,608, respectively.
The actuator 114 moves the objective lens 102 and has a magnetic circuit including a focusing coil, a tracking coil and a magnet (not shown). The objective lens 102 is moved towards and away from the magneto-optical disc 106, based on the focusing error signal Sf supplied to the magnetic circuit; for adjusting the focal point of the objective lens 102. Also, the objective lens 102 is moved along the radius of the magneto-optical disc based on the tracking error signal St supplied to the magnetic circuit, for causing the light beam L from the semiconductor laser 101 to follow the track center.
It, is difficult in actual practice to reproduce the read-only optical disc and the magneto-optical disc by one and the same optical head because these discs are reproduced in accordance with different playback principles. If reproducing signals are to be produced from the read-only optical disc by an optical head for the magneto-optical disc shown in FIG. 1, it may be contemplated to produce the readout signals using one of the first photodetector 103 or the second photodetector 104.
However, with the conventional optical head for reading MO discs, shown in FIG. 1, the two photodetectors 103, 104 need to be arranged at an angle of 90.degree. with respect to each other, thus imposing limitation on the reduction in size of the optical head.